The invention relates in general to the field of filamentary switching resistive memory elements. In particular, it relates to a resistive memory element whose resistively switchable material is an amorphous carbon compound that is laterally confined within a confining material.
Switching resistive memory elements are known. Resistive switching refers to a physical phenomenon occurring in a material that suddenly changes its resistance under action of a strong current or electric field. The change is non-volatile and reversible. Several classes of switching materials (ranging from metal oxides to chalcogenides) have been proposed in the past. The performances of these materials are appreciated mainly in terms of power consumption, integration density potential, retention, and endurance. Typical resistive switching systems are capacitor like devices, where electrodes are ordinary metals and the dielectric a resistive switching material, e.g., a transition metal oxide.
An interesting application of resistive switching is the fabrication of non-volatile resistive random-access memories (RRAM), which are promising candidates to replace conventional flash memories as they offer better scalability, higher integration density, higher throughput, lower access time, and lower power consumption.
Amorphous carbon (aC) has been proposed as a resistive switching material for RRAM applications. Compared to oxide-based RRAM, carbon promises higher memory density and lower power consumption. The mono-atomic nature of carbon would make a carbon-based memory cell scalable, even to single bonds. Such cell dimensions would limit the reset current, thus reducing the power consumption. In addition, the high resilience of carbon would enable operation at high temperatures.
Another intrinsic advantage of aC-based RRAM is the switching mechanism. Amorphous carbon is mainly formed by sp2 bonds (conductive) and sp3 bonds (insulating). When a set voltage is applied across the aC layer, the electric field and the Joule heating induce a clustering of sp2 bonds, bringing the cell into a low resistive state (LRS). When another voltage (reset) is applied across the cell, causing a high current to flow through the sp2 filaments, these filaments break down owing to Joule heating, and the cell returns to a high resistance state (HRS), as illustrated in FIG. 8. No electrochemical reaction is involved: the resistive switching in carbon is unipolar, i.e., the memory can be set and reset by means of voltages of the same polarity. In contrast, resistive switching in oxide-based RRAM occurs owing to the reduction (set) and oxidation (reset) of oxygen vacancies. Therefore, voltages of opposite polarity are needed to set and reset the cell (bipolar switching). Unipolar resistive switching, as it occurs in carbon-based RRAM, simplifies the circuit design of the memory devices, compared to bipolar switching circuits.
Another advantage of carbon-based RRAM is that no “conditioning step” is required, whereas such a step is needed in oxide-based RRAM, which involves the application of a high voltage across the cell to induce a soft breakdown and form the channel in which the filaments will then grow. Because the conditioning voltage is typically much higher than the set voltage, this step might degrade the device endurance and therefore is not desirable.
In the known, aC-based resistive memory elements, the carbon compound is typically laterally confined within a confining material. The latter is typically designed for confining heat into the cell comprising the resistively switchable material, to allow lower power consumption, in operation.